Since colored stainless steel plates are mainly used as building material, they are required to have permissible wide color variation, color consistency or no color shading, and high abrasion resistance in addition to the corrosion resistance inherent to stainless steel.
To meet such requirements, there were proposed prior art techniques as shown below.
(1) Prior art known methods for imparting a wide variety of color tones to stainless steel stock are so called INCO methods primarily based on the use of a mixed solution of sulfuric acid plus chromic acid (see Japanese Patent Publication Nos. 52-32621, 52-25817, and 53-31817). These methods include two steps, "coloring" and "film hardening" steps, which are separately carried out with individual solution compositions, temperatures, and treating conditions. Most products are batchwise manufactured plates.
(2) When stainless steel is dipped in an aqueous solution comprising chromic acid and sulfuric acid, there forms a porous colored film of chromium oxides on the surface. This oxide film, however, is liable to abrasion because of porosity. Known methods for hardening such a colored film to overcome this problem are by effecting electrolysis in an aqueous solution containing chromic acid and a much lower concentration of sulfuric acid than in the coloring solution while setting the stainless steel plate colored by the aforementioned method as a cathode, thereby electrodepositing metallic chromium on the surface, as disclosed in Japanese Patent Publication Nos. 53-31817and 56-24040.
(3) Also disclosed is a method for continuously coloring stainless steel hoops (Japanese Patent Publication No. 60-22065). This method is to produce colored stainless steel strips by a dual step process based on the INCO method using dual solutions, "coloring" and "film hardening" tanks. Control of color tone is accomplished by measuring the potential between the steel strip and a counter electrode, platinum plate at a plurality of positions on the path of the strip in the "coloring" tank during the "coloring" step to compute a potential difference from a reference.
(4) Since the use of such sulfuric acid plus chromic acid solution leads to a great expenditure in the solution treatment required in view of pollution control, another coloring method is known involving dipping in sulfuric acid plus permanganate salt as a hexavalent chromium-free coloring solution (Japanese Patent Publication No. 51-40861). In this method, a dipping solution is prepared by adding a permanganate salt to aqueous sulfuric acid and allowing reaction to proceed until oxygen gas ceases to evolve, and stainless steel is dipped in the solution at a temperature in the range from 90.degree. to 110.degree. C., thereby forming a film colored in bronze, blackish brown or black color.
In addition to these solutions, a variety of coloring solutions have been developed. There is known a method for spontaneous coloring by dipping in a hot solution of sodium (or potassium) hydroxide plus potassium (or sodium) permanganate as one of such solutions (Japanese Patent Publication No. 54-30970).
However, the aforementioned prior art techniques have problems as described below.
The INCO method identified in (1) which consists of two steps, "coloring" and "film hardening" steps has the problems that water rinsing and drying operations must be inserted between the two "coloring" and "film hardening" steps in order to perform them in a continuous fashion; that because of a change of the originally imparted color during the "film hardening" step, the preceding "coloring" step requires a complicated adjustment to take into account the subsequent color change in order that the predetermined color be eventually obtained; and that dipping operations often used in the "coloring" treatment cannot avoid color shading at edges of colored articles.
The process is difficult to perform on an industrial continuous line because it is based on dual solution-dual step of "coloring treatment" and "film hardening treatment" and thus complicated.
The film hardening treatment identified in (2) requires two separate treating tanks for coloring and film hardening steps, and the need for water rinsing and drying between the coloring and film hardening steps makes the process complicated, resulting in color shading and low productivity. The cost of colored stainless steel is thus considerably increased and the use thereof is limited although there is a great potential demand as building materials (including interior and exterior materials).
Since a film hardening treatment solution used is different from a coloring solution, steel stock must be once taken out of the coloring tank before proceeding from the coloring step to the film hardening step. This leads to a problem of impairing aesthetic appearance, for example, occurrence of color shading.
The continuous coloring method identified in (3) accomplishes control of color tone on the basis of a potential difference with respect to a reference, and thus inevitably requires control of dipping time. This results in a complicated and difficult system wherein the speed of transfer of steel strip must be always changed by means of a winding motor. With respect to color tone, it is not easy to obtain products with the predetermined color because the "film hardening" treatment effected as the subsequent step inevitably invites a color change.
The immersion coloring in a mixed aqueous solution of sulfuric acid and permanganate salt identified in (4) suffers from the difficulty of solution maintenance because the process is carried out at a very high temperature of 90.degree. to 110.degree. C. so that the solution undergoes a substantial change of concentration due to evaporation. Evolution of vapors gives rise to a safety and hygienic problem to operators and a large sized exhaust disposal equipment must be installed, causing an increase of cost.
In the method of oxidative coloring with sodium hydroxide and potassium (or sodium) permanganate, the sodium (or potassium) hydroxide is used as an oxidation accelerator because the potassium (or sodium) permanganate alone has a weak oxidizing power. Black dyeing is achieved with immersion for 10 to 20 minutes at a solution temperature of 90.degree. to 130.degree. C. Since spontaneous immersion coloring with potassium (or sodium) permanganate and sodium (or potassium) hydroxide is carried out at a very high temperature of 90.degree. to 130.degree. C., the solution undergoes a substantial change of concentration due to evaporation, leading to difficulty in solution maintenance. Another problem is frequent color shading due to the high temperature treatment. The shortcoming of frequent color shading is critically detrimental to all applications including building and decorative materials. Industrial production cannot be applied unless this problem is solved.